The present invention generally relates to superconducting devices. More specifically, the present invention relates to shadow mask area correction for tunnel junctions in quantum computing applications.
Evaporation is used in microfabrication. Evaporation is a common method of thin-film deposition in which the deposition source material is evaporated in a vacuum. During evaporation, a hot source material evaporates and then condenses back to a solid state on the target object (sample). Because the evaporation takes place in a vacuum, vapors other than the source material are almost entirely removed before the process begins. In a high vacuum (with a long mean free path), evaporated particles can travel directly to the deposition target without colliding with the background gas. At a typical pressure of 10−4 Pascals (Pa), a 0.4 nanometer particle has a mean free path of 60 meters. Evaporated atoms that collide with foreign particles can react with them. For example, if aluminum is deposited in the presence of oxygen, it will form aluminum oxide. Evaporated materials deposit non-uniformly if the substrate has a rough surface (as integrated circuits often do). Because the evaporated material attacks the substrate mostly from a single direction, protruding features block the evaporated material from some areas. This phenomenon is called “shadowing” or “step coverage.”
A common technique for the fabrication of Josephson junctions (JJs) involves double-angle shadow evaporation of aluminum through an offset mask, wherein the tunnel barrier is formed by the diffusive oxidation of the aluminum base layer. Shadow evaporation has been the most successful fabrication approach to date for making long-lived, high-coherence superconducting quantum bits (or qubits).
The Niemeyer-Dolan technique, also called the Dolan technique or the shadow evaporation technique, is a thin-film lithographic method to create nanometer-sized overlapping structures. This technique uses an evaporation mask that is suspended above the substrate. The evaporation mask can be formed from two layers of resist. Depending on the evaporation angle, the shadow image of the mask is projected onto different positions on the substrate. By carefully choosing the angle for each material to be deposited, adjacent openings in the mask can be projected on the same spot, creating an overlay of two thin films with a well-defined geometry. Another technique for fabricating an overlay of two thin films during lithography is known as a Manhattan crossing, named as such because it has intersecting streets and avenues at right angles. In this technique, an evaporation is first done along an avenue at a tilt angle that is typically steeper than that used for a Dolan. This results in deposition of material along the avenue but no deposition within the street because all the material evaporated into the street is intercepted by the evaporation mask such as a resist. The crossing is then formed by performing a similarly steep subsequent evaporation along the street. No material is evaporated within the avenue in the subsequent evaporation because all the material evaporated into the avenue is again intercepted by the evaporation mask. The only area not protected by the evaporation mask is the intercept between the street and the avenue, resulting in a Manhattan crossing overlay.
New shadow mask evaporation techniques are needed to form tunnel junctions, such as Josephson junctions for superconducting quantum computing applications. In particular, new techniques are sought which can reduce variability of fabrication.